Conventionally, sheets made of nonwoven fabrics have often been used as air filters for removing fine dust such as pollen and fine particles. Such an air filter is required to have a capability of highly efficiently collecting dust and have such a low pressure loss that an intake resistance occurring when gas passes through the air filter is small. These air filters make it possible to collect fine dust such as pollen and fine particles by physical action as well as electrostatic attraction and the like by forming a dense matrix of fibers having fine fiber diameters and by electret-processing the filters.
As these air filters, nonwoven fabrics made of filaments having average fiber diameters of 15 μm or less are often used to form the dense matrices.
As nonwoven fabrics used as filters, melt-blown nonwoven fabrics having fine fiber diameters are often employed. A filter made of a melt-blown nonwoven fabric has a fine fiber diameter and hence has a dense structure, and achieves a high collection efficiency of fine dust. Hence, melt-blown nonwoven fabrics are suitably used as materials for filters. However, most filters made of melt-blown nonwoven fabrics are generally low in bulk and paper-like, and are difficult to use as filters for long periods because the pressure loss increases rapidly in the use. Patent Document 1 proposes a bulky composite fiber nonwoven fabric made of thermoplastic composite fibers. This nonwoven fabric is aimed at pleating suitability. The bulk of the composite fiber nonwoven fabric is increased by partially aggregating the fibers during spinning so that a lot of fusion bonded fiber aggregates are made present in the nonwoven fabric. Hence, the fiber aggregates cannot be dispersed uniformly, and a difference in density is created in the nonwoven fabric. Hence, the nonwoven fabric tends to have a low collection efficiency, although the pressure loss is low.
Meanwhile, Patent Document 2 proposes a melt-blown nonwoven fabric being obtained by blending fibers having different diameters and having a low pressure loss and a high collection performance. The melt-blown nonwoven fabric tends to have a lower pressure loss than melt-blown nonwoven fabrics made of fine fibers. However, since the melt-blown nonwoven fabric is a nonwoven fabric made of fibers of a single component resin, the nonwoven fabric has a high density and an insufficiently lowered pressure loss.